When an order is started and an item is added to cart on your eCommerce store, the order number is created, for example 000005.
The customer may then leave the order and come back to it later, or completely abandon it.
Order number 000005 is then taken, the next customer order that is processed is 000006.
When you view your orders in the orders summary page, there will be a gap between 00004 and 00006 because there was an abandoned order.
If the customer comes back to complete order 00005 the next day, the order sequence will show 000004 000006 000005 because the sequence goes by the payment date, and completion of the order.